In general, electrophotographic image forming apparatuses such as copiers, printers, facsimile machines, or multifunction machines including at least two of these functions include a development device to develop latent images formed on an image bearer with developer, and a toner container for containing toner. As the toner in the development device is consumed in image development, fresh toner is supplied from the toner container to the developer in the development device. If mixing of the toner supplied from the toner container and the developer in the development device is insufficient, the ratio of supplied toner in the developer becomes uneven between a portion adjacent to a toner supply inlet formed in the development device and other portions inside the development device. In such a state, it is possible that image density becomes uneven or toner scatters in the backgrounds of output images.
Therefore, developer is typically circulated inside the development device to equalize the ratio of supplied toner in the developer inside the development device (i.e., development housing). For example, U.S. Pat. No. 5,887,224-A, JP-2006-276535-A, and JP-2008-129210-A) propose configurations in which the interior of the development housing is divided into first and second compartments that communicate with each other in both axial end portions, and first and second rotary conveyance screws (collectively, “conveyance screws”) are provided in the first and second compartments, respectively, to transport the developer therein.
The conveyance screws transport the developer in the respective compartments in the opposite directions, and the developer transported to an axial end portion in one compartment is transported through the communication portion to the other compartment. Then, the developer is transported by the conveyance screws in the first and second compartments to the opposite axial end portions, respectively, after which the developer is returned through the communication portions to the compartment where the developer was originally. The developer can be circulated between the first and second compartments by repeating this operation. Accordingly, the supplied toner can be mixed with the developer present in the development device, and the ratio of supplied toner to the existing developer can become uniform.
In such configurations, any contact of the rotary conveyance screw with the inner wall of the development housing can generate noise and cause wear of the conveyance screw as well as the inner wall of the development housing. Therefore, a certain clearance or gap is generally provided between the edge of the conveyance screw and the inner wall of the development housing. The size of the gap includes a margin for error in view of dimensional tolerance of the development housing and the conveyance screw, runout of the conveyance screw, and the like.
A drawback of the above-described arrangement, however, is that the conveyance screw cannot convey the developer in the gap between the conveyance screw and the inner wall of the development housing, allowing the developer to remain in the gap. If the developer thus accumulates locally, the amount of developer circulating in the development device is smaller than the amount of developer contained therein.
Additionally, although toner is degraded gradually as toner particles contact each other while being circulated, when the amount of developer circulating is smaller, the number of times toner particles contact each other increases, thus accelerating degradation of toner. Therefore, allowing developer to accumulate in the gap can accelerate degradation of toner.
In view of the foregoing, various approaches have been tried to prevent developer from accumulating in the development housing.
For example, in JP-2008-129210-A, a flexible agitation blade is configured to slide along the inner wall of the development housing to prevent the developer from adhering to the inner wall of the development housing. Although noise and wear caused by the flexible agitation blade can be reduced compared with the conveyance screw, this approach, however, has a drawback in that the agitation blade is not capable of transporting the developer axially. Accordingly, if the spiral blade of the conveyance screw is replaced with the agitation blade, it is possible that conveyance of developer is degraded in that portion.
Additionally, JP-2008-242266-A proposes a development device in which both a conveyance blade capable of transporting developer axially and an agitation blade to prevent the developer from adhering to the inner wall of the development housing are provided in an identical area. Although this approach is effective, the configuration is complicated and costly.